Riesz transform

In the mathematical theory of harmonic analysis, the Riesz transforms are a family of generalizations of the Hilbert transform to Euclidean spaces of dimension d > 1. They are a type of singular integral operator, meaning that they are given by a convolution of one function with another function having a singularity at the origin. Specifically, the Riesz transforms of a complex-valued function ƒ on R^d are defined by

${\displaystyle R_{j}f(x)=c_{d}\lim _{\epsilon \to 0}\int _{\mathbf {R} ^{d}\backslash B_{\epsilon }(x)}{\frac {(x_{j}-t_{j})f(t)}{|x-t|^{d+1}}}\,dt}$

(1)

for j = 1,2,...,d. The constant c_d is a dimensional normalization given by

${\displaystyle c_{d}={\frac {1}{\pi \omega _{d-1}}}={\frac {\Gamma [(d+1)/2]}{\pi ^{(d+1)/2}}}.}$

where ω_d−1 is the volume of the unit (d − 1)-ball. The limit is written in various ways, often as a principal value, or as a convolution with the tempered distribution

${\displaystyle K(x)={\frac {1}{\pi \omega _{d-1}}}\,p.v.{\frac {x_{j}}{|x|^{d+1}}}.}$

The Riesz transforms arises in the study of differentiability properties of harmonic potentials in potential theory and harmonic analysis. In particular, they arise in the proof of the Calderón-Zygmund inequality (Gilbarg & Trudinger 1983, §9.4).